The present invention relates to the field of telephony, and more particularly to interface circuitry for providing a ring signal and DC power to a telephone subscriber line.
All telephones require an alerting signal or ring signal for notifying a subscriber of an incoming phone call. Early telephones employed mechanical bells that rang in response to an electrical ringing signal appearing on the telephone line. Backward compatibility requirements have kept the characteristics of the ringing signal essentially the same over the last century. The mechanical bells require a low frequency, high voltage AC signal for ringing. The amplitude of the ring signal has to be relatively high, for example 45 volts AC at the phone, and the frequency quite accurate to ring the phone without fail.
In the past, ring generation was handled at the central office, and the ring generator supplied ring signals to many telephone lines. The ring generator was continuously in a single mode (i.e., ringing), and lines were randomly connected to the ring generator when a ring signal was needed. External logic and switching circuitry were used for each telephone line to allow the line to be connected to or disconnected from the ring generator as required.
The ring generator design to meet the above-described requirements is a challenging power conversion problem. Because of the centralization of prior ring generation, this design challenge was confined to a low volume, higher power level application, where efficiency and cost were not always the driving considerations. In recent years, due to proliferation of newer telecommunication networks, the ring generation function has shifted away from the central office, and thus newer designs have been required that are more tailored to the environments in which they operate. At the utmost, ring generation can occur at the individual subscriber or single line level, in which case cost and efficiency become paramount concerns because of the high volumes involved. The industry trends of Fiber-to-the-Home (FTTH), Wireless Local Loop (WLL), ISDN Terminal Adapters, and Cable Set Top Boxes (STB) could all require a local ring generator for interfacing to the Plain Old Telephone System (POTS).
There are circuits being implemented today for ring generation that address both cost and efficiency concerns. However, many of these circuits do not meet existing standards for ring signals. For example, in order to cut costs, the ring signal used is trapezoidal instead of sinusoidal. The harmonic content of the trapezoidal signal generated by these circuits can create interference in adjacent lines or circuits.
This problem has been addressed in a technique described in U.S. Pat. No. 5,663,878 to Walker, presently assigned to Unitrode Corporation. The Walker patent describes a unique circuit for generating a low frequency sinusoidal signal for resonant ring generator applications. The circuit uses a switched DC to AC converter in which a sinusoidal AC output of a transformer is sampled at a frequency differing from the frequency of the sampled signal by 20 Hz, and the sampled output is filtered to obtain a 20 Hz sinusoidal ringing signal. The circuit is suitable for use with a single subscriber line as well as other configurations. Because the circuit generates a sinusoidal ring signal, it has less tendency to induce interference in adjacent telephone lines or circuits than the above-described solutions.
The circuit in the Walker patent does share one characteristic with prior ring generators. Telephone systems require a talk battery voltage, typically -48 volts DC loosely regulated, for each telephone line. The line is connected to the ring generator when a ring signal is needed, and otherwise is connected to the talk battery voltage source. Thus prior ring generators have been used with a separate talk battery voltage source, along with relays and additional logic to switch between the ring generator and the talk battery voltage source for each telephone line. Thus modern subscriber line interface circuits have unfortunately retained some of the circuitry burden from the time when ring generation was performed centrally. As a result, fuller benefits of de-centralizing the ring generation function have not previously been achieved.